The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen," which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid carbonaceous product is called "shale oil." The formation containing oil shale can contain a layer or intermittent layers which are permeable to gases. Exemplary of such a permeable layer is a tuff bed which is composed of compacted volcanic fragments, generally smaller than 4 millimeters in diameter. In the western United States such tuff beds and formation layers containing kerogen are near horizontal.
Formation containing oil shale in the Piceance Creek basin in Colorado has three well defined tuff beds which are persistent throughout the basin. These tuff beds are known as the "curly" tuff bed, the "wavy" tuff bed, and the "mahogany marker" tuff bed. The "curly" bed is about 2 feet thick and is above a mahogany zone, a well defined stratum in the Piceance Creek basin which has a relatively high kerogen content. The "mahogany marker", so named because it is in the middle of the mahogany zone, has a thickness of from about 6 to 8 inches. The "wavy" tuff bed, which is below the mahogany zone, has a thickness of about 2 feet. Other tuff beds of three to four inches in thickness are present throughout the Piceance Creek basin.
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then explosively fragmenting and expanding remaining formation to form a stationary, fragmented, permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the fragmented mass in the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the fragmented mass in the retort and introduction of a gaseous combustion zone feed comprising oxygen into the combustion zone to advance the combustion zone through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the combustion zone feed downwardly into the combustion zone, the combustion zone is advanced downwardly through the retort.
The effluent gas from the combustion zone comprises combustion gas and any gaseous portion of the combustion zone feed that does not take part in the combustion process. This effluent gas is essentially free of free oxygen and can contain constituents such as oxides of carbon, water vapor and sulfurous compounds. It passes through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and to a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. An off gas containing combustion gas generated in the combustion zone, product gas produced in the retorting zone, gas from carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process is also withdrawn from the bottom of the retort to the surface. The off gas can contain constituents such as hydrogen, water vapor, hydrogen sulfide, oxides of carbon, methane, ethane, propane, and other hydrocarbons.
As used herein, the term "processing gas" is used to indicate gas which serves to advance a processing zone such as a combustion zone, a retorting zone, or both a retorting zone and combustion zone, through an in situ oil shale retort, and includes, but is not limited to, an oxygen containing gas introduced into a retort for advancing a combustion zone and retorting zone through a retort and a hot retorting gas which can be introduced into a retort or generated in a combustion zone in a retort for advancing a retorting zone through a retort.
It is desirable for many reasons to know the locus of the combustion and retorting processing zones as they advance through an in situ oil shale retort. One reason is that by knowing the locus of the combustion zone, steps can be taken to control the orientation of the advancing side of the combustion zone. It is desirable to maintain a combustion zone which is flat and uniformly transverse and preferably uniformly normal to the direction of its advancement. If the combustion zone is skewed relative to its direction of advancement, there is more tendency for oxygen present in the combustion zone to oxidize hydrocarbon products produced in the retorting zone and reducing hydrocarbon yield. In addition, with a skewed combustion zone, more cracking of the hydrocarbon products can result. Determining the locus of the combustion zone provides information for control of the advancement of the combustion zone to maintain it flat and uniformly perpendicular to the direction of its advancement to obtain high yield of hydrocarbon products.
Another reason for determining the locus of the combustion zone is so that the composition of the combustion zone feed can be varied with variations in the kerogen content of the oil shale being retorted. If combustion zone feed containing too high a concentration of oxygen is introduced into a region of the retort containing oil shale having a high kerogen content, oxidation of carbonaceous material in the oil shale can generate so much heat that fusion of the oil shale can result. High temperatures also can cause excessive endothermic carbonate decomposition to carbon dioxide and dilution of the off gas from the retort, thereby lowering the heating value of the off gas. Layers in the fragmented mass are correlated with strata in the unfragmented formation because there is little vertical mixing between strata when explosively fragmenting formation to form a fragmented permeable mass of formation particles. Therefore, samples of various strata through the retort can be taken before initiating retorting of the oil shale and assays can be conducted to determine the kerogen content. Then, by determining the locus of the combustion zone as it advances through the retort, the composition of the combustion zone feed can be appropriately modified.
Another reason for determining the locus of the combustion and retorting processing zones as they advance through the retort is to monitor the performance of the retort to determine if sufficient shale oil is being produced for the amount of oil shale being retorted.
Also, by determining the locus of the combustion and retorting zones, it is possible to control the advancement of these two zones through the retort at an optimum rate. The rate of advancement of the combustion and retorting zones through the retort can be controlled by varying the flow rate and composition of the combustion zone feed. Knowledge of the locus of the combustion and retorting zones allows optimization of the rate of advancement to produce hydrocarbon products of the lowest cost possible with cognizance of the overall yield, fixed costs, and variable costs of producing the hydrocarbon products.
Attempts to determine the locus of combustion and retorting processing zones in an in situ oil shale retort with temperature transducers or devices for withdrawing gas samples have encountered problems with corrosion. Not only are high temperatures present in a retort, but also the gases passing through the retort contain corrosive compounds such as water, hydrogen sulfide and other sulfurous compounds. Therefore, temperature transducers and sampling devices placed directly in a retort can fail from corrosion in a matter of weeks, or even days.
Thus, it is desirable to provide a method for determining the locus of combustion and retorting processing zones advancing through an in situ oil shale retort.